There are many commercial applications in which it is desirable to monitor the concentrations of components in gas streams. In particular, it is important for medical personnel to monitor the concentrations of the various components in a patient's respiratory stream to dispense the proper amount of medication and/or identify potentially hazardous conditions. This is especially important in the field of anesthesiology, where gaseous anesthetic or therapeutic agents, such as nitrous oxide, halothane, enflurane, desflurane, sevoflurane, and isoflurane, are dispensed to the patient in controlled dosages. Therefore, monitoring anesthesia may involve analyzing the respiratory stream with respect to one or more components, possibly including anesthetic or therapeutic agents, as well as other respiratory gases, such as carbon dioxide.
One type of instrument that has been employed for monitoring respiratory gases is the spectral gas analyzer. Spectral gas analyzers provide an indication of the presence and concentration of selected components in a gas sample based on the detected spectral composition of illumination transmitted through the gas sample. The gaseous components of interest can be characterized with regard to specific illumination absorption properties. For example, a particular gaseous component may be characterized by an absorption band at a particular wavelength or over a wavelength range. By comparing the intensity of transmitted and received illumination of a selected wavelength or range of wavelengths for a particular gas sample, information regarding the absorption characteristics and composition of the sample can be obtained.
In order to monitor a variety of respiratory gases as well as anesthetic agents and/or therapeutic agents, spectral gas analyzers typically employ infrared illumination. In particular, the components of interest generally have distinctive absorption spectra in the near to mid infrared spectral range. Accordingly, multiple component gas analyzers typically employ either multiple infrared sources or a single broad band infrared source. The resulting polychromatic signals are transmitted through a gas sample and detected by a detector system that is designed to analyze the signals with respect to multiple wavelengths. In this regard, multiple detectors or a single detector array associated with a variable wavelength filter may be employed. In either case, standard chemometric calculations based on the output of the detector system can be used to identify and quantify the gases of the sample so that this information can be reported to the instrument user.
For a number of reasons, multiple component infrared gas analyzers have generally been implemented with a side-stream system architecture. That is, a portion of the respiratory stream is diverted from the patient's respiratory circuit for analysis. First, infrared sources typically generate significant heat. For example, certain broad band sources are operated at a temperature in excess of 900.degree. C. in order to provide the desired illumination intensity across the infrared range of interest. It is, of course, desirable to locate such a heat source at a suitable distance from the patient. Moreover, in order to reduce optical losses and alignment concerns associated with mirrors and other conventional infrared optical elements, it is desirable to minimize the optical path length, and simplify the path configuration, between the source and the sample to be analyzed. Accordingly, various optical considerations have been thought to favor side-stream architectures. Moreover, the detector systems employed for multiple component infrared gas analysis tend to be massive and bulky. Because space in proximity to the patient is generally at a premium during medical procedures, it has been thought advantageous to locate the detector systems and associated processing equipment remote from the patient. Thus, both optical and space-related considerations have led to the acceptance of side-stream architectures for multiple component infrared gas analysis in the medical environment.